Coupled amplifier cascade



y 1941- E. SEVERIN 2,240,580

COUPLED AMPLIFIER CASCADE.

Filed Feb. 4, 195s SOURCE OF v E y I 1 WAl/E ENERGY SOURCE 0E l CONTROLLING 72 POTENT/AL5 1 1 E ==L v ouTP'uT C 5 1 Q IMPEDANCES c F Lu u 1 19-2 2-3 k k E a 12 g u D Q E as 2 I Q I Q -e 62 0 POTENTIAL '-u 2 i5 2 83 m V) i o MODULATION VOL TAGE AWL/E0 70 GRID 0F TUBE 1 Fhg 6" INVENTOR EDWIN SEVER/N ATTORNEY Patented May 6, 1941 CQUPLED AMPLIFIER CASCADE.

Edwin Severin, Berlin-Tempelhof, Germany, assignor to Telefnnken Gesellschaft fur Drahtlose Telegraphic m. b. H., Berlin, Germany, a

corporation of Germany Application February 4, 1938, Serial No. 188,699 In Germany February 6, 1937 2 Claims.

An object of this invention is the reduction of the rattle or blur factor (non-linear harmonic distortion) in case of grid modulation and reduction in size of modulator tubes. ihe object is attained as follows:

In the presence of the radio frequency carrier the grid-current characteristic as a function of the'audio frequency potential follows roughly a square law. The plate-current characteristic of the modulator tube presents a similar shape. Both may be made to coincide or register optionally by the aid of parallel or series resistances. The invention has been tried out in the laboratory and it has been embodied in various instances in practice.

'The invention is concerned with an amplifier cascade in which an input tube controls a tube connected to its output which latter tube, subject to the influence of the driver or control currents, operates in the presence of fluctuating grid cathode impedance. The invention more particularly is concerned with modulation circuit organizations in which, by the aid of a transformer, the grid circuit of a mixer tube wherein modulation takes place is driven from a modulation amplifier tube, referred to hereinafter as a modulator tube. The end or power stage of a modulation amplifier, particularly in the case of grid voltage modulation, is loaded by a consumer or load device, that is, the mixer tube whose resistance is a function of the amplitude of the driving voltage and which is roughly of a shape following a square law. This shape is due to the grid-current characteristic of the mixer tube.

Mixer tubes of this kind have in the past been operated in such a manner that the size of the tubes was chosen as ample as possible, while their internal impedance was chosen low in order that the lack of linearity of the aggregate circuit may turn out as small as feasible, so that, in spite of a variable external resistance, the voltage at the non-linear consumer or load may be practically sinuous in form. To this end,

however, very large mixer tubes have been re-.

quired, and the result has been that equipment of this kind. turned out to be expensive and uneconomical in use, thereby precluding its employment in transportable or portable equipment. The attempt to use smaller mixer tubes,

however, is defeated by inadmissibly high blur or rattle factor (non-linear harmonic distortion).

Now, the solution disclosed in the invention is predicated upon the fact that the grid-current characteristic of the mixer tube 2 and the plate current characteristic of the modulator tube obey the same emission laws. This very fact offers ways and means designed to combat and cure the above distortion effect. Thus, according to the invention the internal impedance of the modulator tube is adapted as far as feasible to the grid-cathode resistance of the tube 2 which represents the consumer or load insofar as the working voltages and the working point t of the modulator tube are so chosen that for each point of the operating characteristic the inner resistance of the modulator tube is entirely or nearly equal to the transferred outer resistance resulting from the transformer-coupling of the grid-cathode path of the tube 2. Such adaptation or matching which will preclude distortions results, in the majority of cases, without the use of distinct auxiliary means. Under certain circumstances it may be necessary, by the use of suitable series or parallel impedances, provided in the plate circuit and/or the load circuit to secure matching between the outside circuit and the inner circuit of the tube. My invention is, of course, not limited to cascaded stages transformer-coupled but is applicable to any relaying stages connected in cascade by circuits which transfer variations in impedance from one stage to the preceding stage or vice versa.

An exemplified embodiment of the invention is represented in Figure l of the drawing. Figures 2 and 3 are curves used in describing the operation of my invention.

Referring to the drawing, I denotes the modulator tube controlled by the voice currents and which through a transformer T1, acts upon the grid circuit of the mixer tube 2. The introduction of the radio frequency in the grid circuit of the tube 2 is insured by the aid of a transformer T2. In parallel relation to transformer T1 a by-pass condenser C is provided for the radio frequency. The requisite grid biasing voltage is furnished from the battery B. The modulation transformer T1 is so designed that the two characteristics Bi and Ba as a function of a will practically come to register. resents the internal resistance of the modulator tube I for the load curve Re. which is the grid to cathode resistance of the modulated tube 2.

Figure 2 shows the grid-current characteristic of the tube 2. The abscissa stands for the grid potential 8g, and the ordinate for the grid current 11;, for the resista ce R2, of the grid-cathode path which is a function of the grid voltage and the grid current. The graph denoted by i illustrates the inter-dependence between thegrid The curve R1 repcurrent and the grid biasing potential; while graph Ra shows the dependence of the resistance of the grid-cathode path of the tube to be modulated in di,

upon the grid biasing potential.

As the grid of tube I is swung more negative by the modulation voltage the internal resistance R1 (the ratio of an increment of plate voltage to the resulting increment of plate current) increases, and vice versa, as is well known. Now

transformer T1 is so poled that the grid of tube 2 also swings more negative as the grid of tube becomes more negative so that Ra also increases, as shown by Fig. 2. In other words, modulation potentials applied to the grid of tube 1 cause both R1 and Re. to vary similarly. This is shown in Figure 3. the shape of Ra, the solid-line characteristic corresponds to the shape of R1. What may be seen from this diagram is that the two resistances roughly have the same form which proves that the occurrence of non-linear distortions has been reduced to a minimum amount. This is because the amplification depends on the ratio of load resistance Ra to internal tube resistance B1 so that the amplification will not vary with input amplitude i. e. there will be no non-linear dis- The dash-line curve corresponds to modulating potentials, said electron discharge amplifier tube having output electrodes, a modulation potential transformer having a primary winding connected to the output electrodes of said electron discharge amplifier tube and having a secondary winding connected to the grid and cathode of said'first named tube, the transformer coupling being so arranged that the internal resistance characteristic of the said amplifier increases when the load impedance impressed by the grid to cathode circuit of said first named tube increases and Vice versa to thereby maintain a constant relation between the voltage across the said grid circuit comprising the non-linear load and the modulating potential impressed on the input electrodes of the electron discharge tube amplifier.

2. In a modulation system, a first tube having a grid and cathode connected in a grid cathode circuit excited by wave energy to be modulated, and havingmodulating potentials impressed on said grid circuit, wherein the ratio of grid .cur-' rent averaged over a radio-frequency cycle to modulation potential impressed increases with increasingly positive modulation potential, a second tube having a grid electrode excited by modulation potentials and having an anode cou pled to said grid circuit of said first tube for impressing said modulation potential on the said grid circuit of said first tube, means for causing the ratio of anode current of said secondtube to grid potential of said second tube to increase with increasingly positive modulation potential on the grid of said second tube, and a transformer in said coupling between the anode electrode of said second tube and said grid circuit EDWIN SEVERIN. 

